1. Field of the Invention
The present invention concerns a laser trapping apparatus that optically, traps a certain particle from a group of particles, such as microorganisms dispersed and suspended in a medium by a laser beam focused at a focal point of an optical converging system.
2. Description of the Prior Art
The optical particle trapping technique has been developed by A. Ashkin, et al (Physical Review Letters, Vol. 54, 1985).
As shown in FIG. 6, two objective lenses are arranged along an identical optical axis around a trapping position as a center, and two laser beams are irradiated along the identical optical axis from both opposing sides of the trapping position thereby trapping a particle 2 in a cell 3, which is called as "trapping by radiation pressure".
Because the two objective lenses are opposed to each other in use, this system is problematic in that the optical axes of the lenses must be aligned accurately to each other at a 1 .mu.m order.
Further, when a mechanical operation is conducted, such as, sucking a particle trapped between each of the objective lenses by a micropipette, the operation must be conducted by inserting the pipette in a narrow gap between each of the lenses. The top end however can not be moved freely, resulting in a problem of poor maneuverability.
In order to overcome such a disadvantage, A. Ashkin et al have proposed a technique of trapping a particle by irradiating only one laser beam instead of irradiating two laser beams to the particle (Optics Letters Vol. 11 No. 5, 1986).
In this technique, as shown in FIG. 7, a laser beam is converged by one objective lens thereby trapping a particle to a focal point, which is referred to as "single-beam gradient force optical trap".
According to this method, since the particle can be trapped by irradiating light only from one direction, there is no requirement of aligning optical axes of two optical systems. Accordingly, if an optical system of an inverting microscope is utilized to invert the irradiation direction, and a laser beam is irradiated for trapping just beneath a cell 3 storing a liquid medium in which particles 2 are dispersed and suspended, a micropipette or the like can be operated freely since the upper surface of the cell is opened. Therefore, this method has attracted an attention as a technique capable of handling particles easily.
This configuration of laser trapping, however, is problematic in that the optical power density at the focal point is extremely high (from 10.sup.6 to 10.sup.7 times of the sunlight on the earth). Therefore the optical power may damage microorganisms when trapped as a particle (refer, for example, to Hong, Liang, et al. Biophysical Journal Vol. 70, 1996).
An objective of laser-trapping a microorganism is to separate only one microorganism from a group of a number of microorganisms.
For this purpose, it is necessary to move one microorganism in a trapped state to a position apart from other microorganisms of the group. Therefore a laser trapping apparatus must have a trapping force capable of enduring movement.
Further, another objective of laser trapping is to irradiate a processing laser beam to a microorganism being trapped by a laser beam to apply processing such as cell fusion. A trapping force capable of enduring the radiation pressure of the processing laser beam is required.
Therefore, it is important that the trapping force is great upon applying operation to the microorganism.
Because the trapping force is in proportion with the optical power, if it is intended to obtain a large trapping force, the optical power must be increased by so much. The increase of the optical power, however, results in a problem of damaging the microorganism.
In view of the above, it is a technical objective of the present invention to obtain a large trapping force without increasing the optical power.